The present invention relates, in general, to the treatment of defects in the human heart and, in particular, to destroying aberrant electrical pathways in the myocardium of the heart by creating lesions in the myocardium by ablation of the myocardium.
Catheter ablation of cardiac tissue has become widely used in the treatment of symptomatic arrythimias. Cardiac arrythmias occur in areas of the cardiac muscle that have abnormal electrical conduction. This disturbs the propagation of the electrical signals resulting in abnormal muscle contraction. This, in turn, manifests itself in atrial fibrillation or ventricular tachicardia, among other conditions.
It is well known that these arrythmias can be eliminated by interfering with the aberrant electrical conduction pathways that initiate the contraction process. Irreversibly damaging or ablating the pathological tissue regions is one way to achieve this result.
At the present time, however, the only true known cure for atrial fibrillation is a highly invasive surgical procedure known as the xe2x80x9cmazexe2x80x9d procedure. In the maze procedure, a transmural barrier to aberrant electrical conduction is formed surgically. This procedure, while curative, is so invasive that relatively few are performed annually worldwide.
Currently, the predominant means for generating ablative lesions in the heart is through the use of radio frequency energy. During this type of procedure, the arythmogenic area is mapped and radio frequency energy is delivered from the tip of a catheter that is inserted percutaneously into an artery or a vein. Energy is deposited in the tissue through ionic conduction of electricity, which generates heat in the process. The energy is thermally conducted through the tissue from the source, at the point of contact, in all directions. In the process of creating transmural lesions, volumes of ablated tissue are generated that are far greater in width than necessary or desirable.
For the past five years, electrophysiologists and catheter companies have been working to develop catheters to duplicate the maze procedure or some other set of lesions without the need for open heart surgery. This effort, while still underway, has been largely unsuccessful for a number of reasons, including inadequate contact of the ablating electrode with the myocardium, and technical difficulty of the procedure in creating full thickness (i.e., transmyocardial) continuous lesions.
The desired result of any such ablation equipment is the creation of a lesion deep enough into the myocardium to destroy aberrant electrical pathways, yet limit the amount of collateral damage done to healthy tissue.
Ultrasound offers an energy source that, in many respects, is very well suited for ablation of cardiac tissue in the treatment of symptomatic arrythmias. Because ultrasound is transmitted effectively in both blood and fat, an ultrasound ablation transducer need not be in intimate contact with the myocardium being treated. Also, because ultrasound energy is transmitted as an acoustic wave through most solid and liquid media, ultrasound energy can be focused and manipulated in much the same way as light allowing for the delivery of maximum energy within the cardiac wall.
An important characteristic of acoustic wave transmission in tissue, especially at high frequencies (e.g., 1 to 20 MHz), is that some energy is absorbed as the wave propagates through the tissue. This property permits using acoustic energy to selectively heat tissues to a temperature where the proteins are denatured and cells irreversibly destroyed. It has been demonstrated that by applying ultrasound energy to cardiac tissue for extended periods of time (e.g., up to sixty seconds), tissue can be heated and an ablation lesion can be produced.
A method for ablation of cardiac tissue in a myocardium, performed in accordance with the present invention, includes the steps of locating a region of aberrant electrical pathways in the myocardium and scanning a focused ultrasound beam through the region of the myocardium to destroy aberrant electrical pathways by creating a lesion of desired extent in the myocardium.
Apparatus for ablation of cardiac tissue in a myocardium, constructed in accordance with the present invention, includes an ultrasound transducer and control means for energizing the ultrasound transducer to transmit a focused ultrasound beam at selected times and for selected periods to selected points in the myocardium to destroy aberrant electrical pathways in the myocardium by creating a lesion of desired extent in the myocardium.